Electrode system and method of making same



Jam; 13, 1939. H, EMMEN HAL 2,162,613

ELECTRODE SYSTEM AND METHOD OF MAKING SAIIE Filed Aug. 9', 1937 m/zwroms mvzw/r EMNEMS Arrvmvzr.

Patented June 13, 1939 UNITED STATES .ELECTRODE SYSTEM AND DIETHOD OF MAKING SAME Hendrik Emmens and Willem Christiaan van Geel, Eindhoven, Netherlands, assignors to N. V. Philips Gloeilampenlabrieken, Eindhoven, Netherlands Application August 9,

In Germany August 13, 1936 11 Claims.

Our invention relates to asymmetrically-conductive electrode systems of the type in which an insulating intermediate or blocking layer sepa rates a selenium electrode from a second electrode of higher conductivity.

To improve the rectifying ratio of such systems it has been proposed to add to the selenium, admixtures such as sulphur or earth metals.

The main object of our invention is to improve the properties of the insulating intermediate layer of such systems while at the same time retaining the advantages of admixtures in the selenium electrode, these advantages consisting in an increase of the conductivity of the selenium electrode and consequently a decrease of the losses in the electrode system.

A further object of our invention is to provide an improved method of making such electrode systems.

In accordance with the invention we form the intermediate or blocking layer at least partly .of a stable, solid, insulating chemical compound which is produced by chemical conversion from a substance incorporated as an addition in the selenium electrode. As additions to the selenium we prefer to use halides which can be readily hydrolyzed with water to produce insulating, stable, solid metal oxides which form at least part of the blocking layer.

By selecting additions which themselves decrease the resistivity of the selenium electrode, we obtain in a very simple manner an insulating coating or blocking layer of excellent prop erties. More particularly, when using non-volatile halides of the above type we obtain a very satisfactory insulating coating; however when using readily-volatile halides we obtain the additional advantage that the selenium electrode is given a favorable conductivity. Furthermore, we may use as additions a non-volatile halide for forming the insulating layer together ,with a readily-volatilehalide for increasing the conductivity. In this case, of course, it is unnecessary that the readily-volatile halide be one which yields, by hydrolysis, an insulating, stable, solid metal oxide.

An electrode system according to the invention has the great advantage that the material of the insulating intermediate layer is highly independent of the main material, i. e., the selenium, of the electrode of lower conductivity. In spite of this, an insulating layer in the form of a thin, coherent, adhering layer can be obtained at the surface of this electrode by chemical conversion of the additions at the surface thereof. Furthermore, as the material used to form the insulating layer is present only as an addition with the selenium, the forming of a semi-conductive electrode and the forming of an insulating coating are functions of difierent ma- 1937, Serial No. 158,264

terials which are already assembled to form an electrode in building up the system. This not only simplifies the manufacture, but also allows the use in known manner of admixtures for increasing the conductivity of the selenium.

In order that the invention may be clearly understood and readily carried into effect, we shall describe same in more detail with reference to the accompanying drawing, in which the single figure is a sectionized fractional view of a dry rectifier according to the invention.

The rectifier shown in the drawing comprises a supporting plate I of aluminum provided with a carbon layer 2, a semi-conductive electrode 3 containing selenium together with additions, for instance zirconium chloride, an insulating blocking layer 4 formed by hydrolysis from the additions in electrode 3 and containing zirconium oxide, and a conductive electrode 5.

In manufacturing such a rectifler by the method of the invention, we add to selenium as additions and in a quantity of less than 10% and preferably about 1% by weight, a substance which is capable of being chemically converted into a stable, solid, insulating chemical compound. For this purpose we use halides, which readily hydrolyze with water to form insulating, stable, solid metal oxides.

While non-volatile halides of the above type will produce a suitable insulating layer, we prefer to use readily-volatile halides because they have the additional advantage of increasing the conductivity of the selenium electrode. In addition, as has been stated, we may use both a non-volatile halide and a readily-volatile halide.

The effect exerted by the addition of readilyvolatile halides on the resistivity of the selenium readily appears from the following, which gives the resulting values of the volume resistivity (p) of selenium provided with about 1% by weight of the listed admixtures. Selenium without admixtures normally has a volume resistivity of from 100,000 to 150,000 ohm cm.

Moreover, the readily-volatile halides listed in Table I readily hydrolyze, and may consequently similarly to many other analogues readily-volatile halides, such as AlCh, men, 81014, AlBrs, ZrBn, 1114, A113, etc. also. be advantageously used both for forming the insulating coating and for increasing the conductivity of the selenium electrode.

In contradistinction to the above, the addi- -tion, for instance WCls reduces the volume resisticity of the selenium to about 66 ohm/cm, whereas the addition of VCls reduces it to about 400 ohm/cm However, when using such halides no insulating oxide is formed during hydrolysis so that in this case another halide capable of being hydrolyzed to form an insulating oxide must be used for forming the insulating coating. In such cases one of the halides of Table I, or a difiicultly-volatile halide which is capable of being hydrolyzed to form an insulating oxide, such as ThCh, B6012, MgBrs, MEI: 01 T1114, must be used.

Although the particular influence of the halides upon selenium cannot be fully accounted for, the fact that they have the property of. distributing in a very fine state throughout the selenium probably plays an important part. The halides referred to above can be readily incorporated in the selenium in known manner, more particularly in such manner that the components of the halide are both dissolved separately in different quantities of moltenselenium. Then both the quantities of selenium are intermixed. The components come into contact with each other and chemically react, to form halides in the selenium. These halides are divided by this method in an extremely fine state.

However, we prefer to mix the additions with the selenium before the latter is purified, for instance by a distillation method. For example, we melt amorphous selenium in an evacuated vessel of Jena glass and then mix therewith a highly-volatile halide, for example zirconium chloride (ZrCh), whichreadily hydrolyzes to yield an insulating, stable, solid metal oxide. To cause the selenium together with the halide admixture to distill over, the mixture is heated to about 400 C. The selenium is then converted into its conductive crystalline modification by heating the mixture for some time, for instance for 10 minutes at a temperature of 210 C. The resistivity of the resulting selenium mixture was found to be about 130 ohm/cm If highly-volatile additions such as TiCh, ZrCh, etc. are used, it is advisable to mix them with the selenium in a. special manner, and in accordance with the invention this is effected by passing through molten selenium a current of gas containing the vaporized addition. For example, we distill normal commercial selenium and remove any admixture which may exert a detrimental influence upon the conductivity. A current of gas such as air, nitrogenium is dried by passing it through washing vessels containing strongly concentrated sulphuric acid, and the dried gas is then passed through a vessel contaming a very volatile halide such astitanium chloride (TiCl4) ,whichvaporizes into tita um vaporwhich is absorbed by the current of gas. This gaseous mixture is then passed through the molten seleniur'n, whereby the gas escapes and most of the titanium chloride is dissolved in the selenium, so that a fine .distribution of the admixture is assured.

A thinlayer of the selenium with the admixtures therein is thereupon applied to the carbon layer 2, which-is used to make the selenium better adhere to the aluminium support I, The can bon is preferably applied to support I from a suspension, for instance by electrophoresis, whereupon it is heated for about 10 to 15 minutes in an oven at a temperature of about 600 C. to make the carbon thoroughly dry and form a coherent layer on the aluminium support. To ensurethat the carbon layer 2 has a smooth surface to which the selenium will firmly adhere, its surface is rubber with a rotating brush to remove excess carbon particles.

The applied layer of selenium with the additions therein is then pmsed to the desirable thickness. To ensure that the selenium will have a very smooth surface, and to prevent the selenium from sticking to the pressing surface, as well as to prevent impurities from penetrating into the selenium, the plate I and the applied layer 2, together with the applied selenium layer, is placed between two mica plates upon which a pressure is exerted. To ensure uniform pressure distribution, it. is preferable to place a plate of resilient material, such as asbestos, upon the free surface of the mica plate in contact with the selenium.,

The next step in constructing the electrode system according to the invention is to convert the halide present at the surface of the selenium layer into a solid insulating metal oxide which forms part of the insulating coating 4. This may be eflected by bringing water, for instance in the form of water vapor or steam, into contact with said surface of the selenium layer while heating same to a temperature of about 200 C. Thereby the halidepresent as an addition in the selenium layer is converted into the oxide, 1. e. zirconium oxide and the hydro-halogen produced is vaporized.

For a. predetermined conversion of the halide it is desirable to provide a deflite quantity of water on the surface and to heat subsequently. Since, however, the greater part of this water volatilizes before reacting with the halide, we prefer to mix the waterwith a solvent which has a higher boiling point than water and which can be spread in a suitable manner upon the selenium-halide layer. Forthis purpose a mixture of 4 parts of glycerin and 1 part of water can be applied in the form of a viscous liquid over a largesurface area to form a layer having a small thickness, for instance about 0.1 micron. After heating same in the above manner the oxide remains in the surface layer, whereas water, glycerin and hydro-halogen have been volatilized. 1

A still better conversion, which penetrates to having an alkaline reaction may be used. For

the abovementioned reasons we prefer to use also in this case a solvent which has a high boiling point and which can be readily spread on the selenium, for example glycerin.

Upon heating, the halide is converted into oxtion, such as pyridine, aniline or quinoline may also be advantageously used to convert the additions into the insulating compound. When using quinoline, however, it is unnecessary to use a solvent having a high boiling point. The use of the other organic substances having an alkaline reaction has the advantage that the inorganic conversion products need not be removed by washing. However rinsing may be desirable also in this case in order to rem ve excess selenium particles from the surface la er.

The substance having an alkaline reaction, or water-if desired mixed with a solvent having a high boiling point-may, for instance, be applied by providing these materials on the above-mentioned mica-plate which is placed in contact with the selenium. The assembly is then heated for some time, for instance for about 10. minutes at a temperature of about 200 C., to convert the halide.

The conductive electrode 5 having a thickness of, for instance 50 to 100 microns, and consisting, for instance of an alloy of bismuth, cadmium and tin having a melting point of about 103 C., is then provided upon the insulating coating 4 by spraying. However, electrode 5 may be of other conductive metallic materials such as iron, brass, aluminum, tin, or bismuth, or alloys thereof. If desired an additional blocking layer (not shown) consisting, for instance of artificial resin, may be applied.

In a dry rectifier, as shown, in which zirconium chloride is used as addition in the selenium layer 3 and zirconium oxide is formed in layer 4, the following results have been obtained for an active surface of 10 sq. cm.: At a voltage of 2 volts the rectified current had an intensity of at least 3 amp, whereas the rectifying ratio still amounted to 1:100 at an inverse voltage of 15 to 20 volts.

While we have described our invention in connection with dry rectifiers and withspecific materials, we do not wish to be limited thereto but desire the appended claims to be construed as broadly as permissible in view of the prior art.

What we claim is:

1. An asymmetrically-conductive electrode system comprising an electrode of a mixture of selenium and a halide which is readily hydrolyzable with water to form a stable, solid, insulating metal oxide, a second electrode of a material of different conductivity from said first electrode, and an intermediate blocking layer containing a metal oxide formed from the halide present at the surface of said first electrode.

2. An asymmetrically-conductive electrode system comprising an electrode of a mixture of selenium and an easily-volatile halide readilyhydrolyzable with water to form a stable, solid,

insulating metal oxide, a second electrode of a material of different conductivity from said first electrode, and an intermediate blocking layer containing a metal oxide formed from said halide present at the surface of said first electrode.

3. An asymmetrically-conductive electrode system comprising an electrode of a mixture of selenium, a readily-volatile halide, and a nonvolatile halide readily hydrolyzablewith water to form a stable, solid, insulating metal oxide, a second electrode of a material of different conductivity from said first electrode, and an intermediate blocking layer containing a metal oxide formed from said non-volatile halide present at the surface of said first electrode.

4. A method of making an asymmetrically conductive electrode system, comprising the steps of mixing with molten selenium a material capable of being chemically converted into a stable, solid, insulating chemical compound, distilling the resulting mixture to purify the same, forming a layer of said mixture, chemically converting said material at the surface of the layer to form a blocking layer containing said chemical compound, and providing; a second electrode upon said blocking layer.

5. A method of making an asymmetricallyconductive electrode system comprising the steps of passing a dry gas over a highly-volatile halide capable of being chemically converted into a stable, solid, insulating chemical compound to form a gaseous mixture, passing the gaseous mixture through molten selenium to dissolve the halide therein, forming a layer of the seleniumhalide mixture, chemically converting the halide at the surface of the layer to form a blocking layer containing said chemical compound, and providing a second electrode upon said blocking layer.

6. A method of making an asymmetricallyconductive electrode system, comprising the steps of mixing with selenium a halide capable of being chemically converted into a stable, solid, insulating metal oxide, pressing the mixture on a conductive support to form a layer, applying to the free surface of the layer while heating, a substance having an alkaline reaction to convert the halide at the surface of the layer into an insulating metal oxide forming a blocking layer, and providing a second electrode upon the blocking layer.

'7. A method of making an asymmetricallyconductive electrode system, comprising the steps of mixing with selenium a halide capable of being chemically converted into a stable, solid, insulating metal oxide, forming a layer of the mixture on a metal support, forming upon the surface of the selenium-halide layer a thin layer of a substance having an alkaline action to convert the halide at the surface of the layer into an insulating metal oxide forming a blocking layer, heating the layer to convert the selenium into its crystalline modification, and providing a second electrode upon said blocking layer.

8. A method of making an asymmetricallyconductive electrode system, comprising the steps of mixing with selenium a halide capable of being chemically converted into a stable, solid, insulating metal oxide, forming a layer of the mixture on a metal support, applying to the surface of the selenium-halide layer a thin layer of a solution of a substance having an alkaline action and a solvent having good spreading properties to convert the halide at the surface of said first layer into an insulating metal oxide forming a blocking layer, heating the layer to convert the selenium into its crystalline modification, and providing a second electrode upon said blocking layer.

9. A method of making an asymmetricallyconductive electrode system, comprising the steps of mixing with selenium a halide capable of being chemically converted into a stable, solid, insulating metal oxide, forming a layer of the mixture on a metal support, applying to the surface of the.

selenium-halide layer a thin layer of a solution of caustic soda lye and a high-melting-point solvent to convert the halide at thesurface of said first layer into an insulating metal oxide forming a blocking layer, heating the layer to convert the selenium into its crystalline modification, and

providing a second electrode upon said blocking layer.

A method of making an asymmetricallyconductive electrode system, comprising the steps of mixing with selenium a halide capable of being chemically converted into a stable, solid, insulatin'g metal oxide, forming a layer of the mixture on a metal support, applying to the surface of the selenium-halide layer a thin layer of an organic base having a high boiling point to convert the halide at the surface of said first layer into an insulating metal oxide forming a blocking layer, heating the selenium layer to convert the selenium into its crystalline .modification, and providing a second electrode upon said blocking layer.

11. A method of making an asymmetricallyconductive electrode system, comprising the steps of mixing with selenium a halide capable of being chemically converted into a stable, solid, insulating metal oxide, forming a layer of the mixture on a metal support, applying to the surface of theselenium-halide layer a thin layer of (mineline to convert the halide at the surface of said first layer into an insulating metal oxide forming a blocking layer, heating the layer to convert the selenium into its crystalline modification, and providing a second electrode upon said blocking layer. I

HENDRIK EMMENS. WIILEM CHRISTIAAN VAN GEEL. 

